Despite the severity and frequency of chronic pain, current medicine lacks diagnostic tools for treatment selection to consistently and effectively alleviate pain on an individual basis. The symptoms of hyperalgesia and allodynia are helpful for clinical diagnoses, however, more subtle temporal changes in sensitivity may also represent important diagnostic endpoints. Painful aftersensations, the abnormal persistence in pain long after the end of a tactile / thermal stimulus, are frequently observed in chronic pain and may be related to altered temporal processing of nociceptive information due to disrupted pain modulation. In order to better understand how nociceptive processing is temporally altered in clinical states, a combination of psychophysical and fMRI studies will examine healthy subjects and CRPS (complex regional pain syndrome) patients to address the following specific aims: Aim 1: To identify temporal alterations in pain perception as a consequence of central sensitization. Aim 2: To identify neuropharmacological substrates supporting temporal processing of noxious stimuli. Aim 3: To identify neurophysiological correlates of temporally altered pain perception in chronic pain. Offset analgesia is a phenomenon that may represent the dynamic engagement of endogenous anti- nociceptive mechanisms during the processing of temporal dimensions of noxious stimuli. Assessment of offset analgesia and subjects/patients responses to acute painful heat stimuli with variable durations and slow fall rates will allow us to elucidate how temporal aspects of pain processing are altered in states of central sensitization (capsaicin-heat sensitization and chronic pain) and pharmacological manipulation (opioid agonist and antagonist). We will then correlate these changes in psychophysical responses to observed physiological abnormalities using functional imaging of cortical and subcortical regions with BOLD (blood oxygenation level dependent) and ASL (arterial spin labeling) fMRI designs. The proposed research in this fellowship application aims to resolve the lack in understanding of pain mechanisms and how nociceptive processing is dramatically altered in chronic pain patients. The investigation of mechanisms that support temporal processing of noxious information may contribute to and improve clinical assessment and treatment of pathological pain states.